The present invention relates to a method for producing a single crystal by melting a polycrystal raw material and pulling a seed crystal from the melt of polycrystal according to the Czochralski method, and a pulling apparatus therefor.
Conventionally, as for the method of obtaining single crystal materials such as semiconductor silicon single crystals, there is widely used a method for obtaining them as cylindrical single crystals by the Czochralski method (referred to as xe2x80x9cCZ methodxe2x80x9d hereinafter).
This CZ method is a method for producing a single crystal by melting a polycrystal raw material and pulling a seed crystal from the melt of the polycrystal. As the method for pulling a single crystal, there are a method using a shaft and a method using a wire.
In recent years, as the diameter of silicon single crystals produced by the CZ method becomes larger, the weight of the crystals is getting heavier. For such crystals of a heavier weight, there have been proposed to safely pull crystals without dislocations by improvement of load carrying capacity of apparatuses, use of necking with a thick diameter, novel methods without using necking, methods of mechanically holding crystals and so forth.
In such CZ method in which the pulling is attained with a wire, a tungsten wire, stainless steel wire or molybdenum wire of superior heat resistance and high hardness is used. As for such a wire, a thicker one is usually used as the crystals become to have a heavier weight.
By the way, it is known that tensile strength of a wire will be reduced as the temperature is increased. However, in the conventional pulling of single crystals of a small diameter and a low weight, there is almost no possibility of causing accidents such as breakage of the wire and drop of the single crystal, because, when load applied to the wire becomes large, distance between the wire and the melt becomes large and hence the temperature of the wire end is decreased.
In contrast, if single crystals of a large diameter and a heavy weight are pulled repeatedly, in the case of a tungsten wire, for example, degradation of the material and partial breakage of the wire has come to be generated.
According to the values mentioned in manufacturer""s catalogs, tensile strength of tungsten wires of 240 kgf/mm2 at ordinary temperature is decreased to 100 kgf/mm2 at 1000xc2x0 C., and it is decreased almost linearly.
According to the experiments of the inventors of the present invention, it was found that, if a wire was repeatedly used under a condition where the temperature of the wire becomes higher than that observed in the conventional pulling condition for single crystals of a small diameter, the wire was degraded and its strength was markedly reduced due to the high temperature. According to actual measurement, the temperature at which the degradation of the wire material begins is 1200xc2x0 C. or more. In the conventional pulling of single crystals of a small diameter, the temperature scarcely exceeds this temperature. Even when the temperature exceeds that temperature, it is only for a short period of time, and thus degradation or strength reduction is scarcely observed.
However, as the diameter of crystals becomes larger, temperature distribution in furnaces of single crystal pulling apparatuses is changed and becomes different from that used for the conventional pulling of crystals of a small diameter, and the time during which the wire is exposed to a high temperature has become longer. Therefore, even if a wire is used in the same manner as the conventional method, the influence of heat tends to become larger and degradation of the wire has come to be observed.
The present invention was accomplished in view of such problems as mentioned above, and its major object is to provide a method for pulling a single crystal wherein temperature around an end of a joint part of a wire and a seed crystal holder is controlled so as not to exceed the temperature at which the degradation of the wire material begins during the period of from seeding to an early stage of the pulling, and a pulling apparatus therefor.
The present invention was accomplished in order to achieve the aforementioned object, and it provides a method for producing a single crystal by pulling the single crystal with a wire according to the Czochralski method, wherein temperature around an end of joint part of the wire and a seed crystal holder is controlled so as not to exceed 1200xc2x0 C. at any time.
In the pulling of a single crystal by using a wire according to the CZ method, if the temperature around an end of a joint part of the wire and a seed crystal holder (also referred to as xe2x80x9ctemperature of a wire endxe2x80x9d hereinafter) is controlled so as not to exceed 1200xc2x0 C. at any time during the period of from the seeding as an early stage of the pulling to necking including the period where the end of the wire most closely approaches to the silicon melt surface as described above, the degradation of the wire material and the reduction of its tensile strength can be prevented, the seed crystal can be surely and safely held, and it becomes possible to stably pull a growing single crystal of a heavy weight.
In the above method, the material of the wire can be one selected from tungsten, stainless steel and molybdenum.
These metals and alloy show relatively little degradation of material and little reduction of strength even at high temperature under an inert gas atmosphere, and sufficiently function as a wire for pulling single crystals. In particular, their degradation can surely be suppressed at a temperature of 1200xc2x0 C. or lower as defined above.
The present invention further provides a method for producing a single crystal, wherein length or lengths of a seed crystal holder and/or a seed crystal is/are adjusted according to temperature distribution in a furnace so that temperature of a wire end should not exceed 1200xc2x0 C.
If the length or lengths of a seed crystal holder and/or a seed crystal is/are adjusted according to temperature distribution in a furnace, so that temperature of a wire end should not exceed 1200xc2x0 C. at any time as described above, degradation of the material as well as the reduction of tensile strength can be prevented, the seed crystal can safely and surely be held, and a growing crystal of a heavy weight can stably be pulled.
For example, temperature distribution in a furnace for pulling a single crystal having a large diameter is different from temperature distribution in a furnace for pulling a single crystal having a small diameter, and the temperature in the region above the melt for the former reaches a temperature higher than that observed in the pulling of a single crystal having a small diameter by several tens degrees to 100xc2x0 C. or more. A wire exposed to such a temperature becomes likely to suffer from degradation of material or reduction of strength. Therefore, if the length or lengths of a seed crystal holder and/or a seed crystal is/are adjusted to be extended so that temperature of a wire end should not exceed 1200xc2x0 C. at any time including the time when it most closely approaches to the melt surface during the seeding, degradation of the material as well as the reduction of tensile strength can be prevented, the seed crystal can safely and surely be held, and a growing crystal of a heavy weight can stably be pulled.
The present invention further provides an apparatus for pulling a single crystal according to the CZ method comprising a seed crystal holder jointed to a wire and a seed crystal, wherein length or lengths of the seed crystal holder and/or the seed crystal is/are adjusted according to temperature distribution in a furnace so that temperature of a wire end should not exceed 1200xc2x0 C.
If there is used such an apparatus for pulling a single crystal comprising a seed crystal holder jointed to a wire and/or a seed crystal of which length or lengths is/are adjusted according to temperature distribution in a furnace so that temperature of a wire end should not exceed 1200xc2x0 C., temperature of a wire end does not exceed 1200xc2x0 C. at any time including the time when it most closely approaches to the melt surface during the seeding, and thus the apparatus can be an apparatus for pulling a single crystal that can prevent the degradation of the material as well as the reduction of tensile strength, can safely and surely hold the seed crystal, and can stably pull a growing crystal of a heavy weight.
The present invention also provides a method for producing a single crystal by pulling the single crystal with a wire according to the Czochralski method, wherein temperature around an end of joint part of the wire and a seed crystal holder is controlled so as not to exceed 800xc2x0 C. at any time.
In the pulling of a single crystal by using a wire according to the CZ method, if temperature around an end of a joint part of the wire and a seed crystal holder is controlled so as not to exceed 800xc2x0 C. at any time during the period of from the seeding of the early stage to necking including the period where the end of the wire most closely approaches to the silicon melt surface as described above, the degradation of the wire material and the reduction of its tensile strength can substantially be avoided, the seed crystal can be surely and safely held, and it becomes possible to stably pull a growing crystal of a heavy weight.
The present invention further provides a method for producing a single crystal, wherein length or lengths of a seed crystal holder and/or a seed crystal is/are adjusted according to temperature distribution in a furnace, so that temperature of a wire end should not exceed 800xc2x0 C.
If the length or lengths of a seed crystal holder and/or a seed crystal is/are adjusted according to temperature distribution in a furnace, so that temperature of a wire end should not exceed 800xc2x0 C. at any time as described above, the degradation of the material as well as the reduction of tensile strength can substantially be avoided, the seed crystal can safely and surely be held, and a growing crystal of a heavy weight can stably be pulled.
The present invention further provides an apparatus for pulling a single crystal according to the CZ method comprising a seed crystal holder jointed to a wire and a seed crystal, wherein length or lengths of the seed crystal holder and/or the seed crystal is/are adjusted according to temperature distribution in a furnace so that temperature of a wire end should not exceed 800xc2x0 C.
If there is used such an apparatus for pulling a single crystal comprising a seed crystal holder jointed to a wire and/or a seed crystal of which length or lengths is/are adjusted according to temperature distribution in a furnace so that temperature of a wire end should not exceed 800xc2x0 C., temperature of a wire end does not exceed 800xc2x0 C. at any time including the time when it most closely approaches to the melt surface during the seeding, and thus the apparatus can be an apparatus for pulling a single crystal that can substantially avoid the degradation of the material as well as the reduction of tensile strength, can safely and surely hold the seed crystal, and can stably pull a growing crystal of a heavy weight.
According to the present invention, by controlling temperature around an end of joint part of the wire and a seed crystal holder so as not to exceed 1200xc2x0 C., preferably 800xc2x0 C., at any time, possibility of causing accidents such as breakage of the wire and drop of the single crystal is substantially eliminated, which are the problems observed in the pulling of recently used single crystals of a large diameter and a heavy weight. Therefore, holding of a seed crystal using a wire is made safe and sure, and a single crystal can be stably and efficiently pulled even when a growing single crystal having a large diameter and a heavy weight is held.
Moreover, it becomes possible to use a wire repeatedly and its renewal frequency is reduced. Therefore, safety of the operation can be increased, and the cost can simultaneously be improved markedly.